The present invention relates to a red phosphor for low-voltage electron beams and more particularly to a red phosphor, for low-voltage electron beams, using a red phosphor (SrTiO3:Pr, Al).
A vacuum fluorescent display is frequently used as a self-emission type display for displaying a predetermined pattern or graphic design on a display portion of an audio system, a household appliance, a measuring instrument, a medical appliance, and the like. The vacuum fluorescent display is also used as light sources of a back-light, a printer head, a facsimile, a copying apparatus, and the like.
Of phosphors for use in these vacuum fluorescent displays, conventional red phosphors ((Zn, Cd) S:Ag, Cl) contain cadmium (Cd). To preserve environment, the following red phosphors for low-voltage electron beams not containing Cd have been developed in recent years: the phosphor (U.S. Pat. No. 2,746,186) composed of the phosphor component containing the oxide of titanium (Ti) and one element selected from among magnesium (Mg), strontium (Sr), calcium (Ca), and barium (Ba) asitsmatrix to which the group III element is added and the protection film, consisting of the oxide, for protecting the phosphor component from a carbon-containing gas; the phosphor (U.S. Pat. No. 2,904,106) in which at least one substance, selected from PtO2 and/or RuO2, is added to the phosphor component containing SrTiO3 as its matrix; the phosphor (Japanese Patent Application Laid-Open No. 2004-75907) in which the surface of the phosphor component containing SrTiO3 as its matrix is coated with the conductive oxide, and the oxide of the platinum group is spread over the surface of the conductive oxide; the phosphor (Japanese Patent Application Laid-Open No. 2004-75908) containing zeolite particles; the phosphor (Japanese Patent Application Laid-Open No. 8-85788) in which the rare earth element and the group III element are added the matrix consisting of the oxide of the alkaline earth metal and Ti; the phosphor (Japanese Patent Application Laid-Open No. 8-283709) in which the protection film is formed on the surface of the matrix consisting of the alkaline earth metal and the oxide; the phosphor (Japanese Patent Application Laid-Open No. 9-87618) in which the high-resistant phosphor component and the low-resistant phosphor component are mixed with each other to allow emission owing to a collision of electrons accelerated at an anode voltage not more than 2 kV; the phosphor (Japanese Patent Application Laid-Open No. 9-255952) in which the substance, having an oxidizing action, is added to the phosphor component having SrTiO3 as its matrix; the phosphor (Japanese Patent Application Laid-Open No. 10-261371) in which the diamond-liked carbon film is formed on the surface of the phosphor component containing SrTiO3 as its matrix; the phosphor (Japanese Patent Application Laid-Open No. 10-279933) in which the group 4b element is added to the phosphor component containing SrTiO3 as its matrix; and the phosphor (Japanese Patent Application Laid-Open No. 10-273658) in which the phosphor component containing SrTiO3 as its matrix is coated with the film consisting of the substance containing Si except SiO2.
However, SrTiO3:Pr, Al serving as the red phosphor, for low-voltage electron beams, not containing Cd reduces at a high rate in its luminance with the passage of time and thus has a short life. In an operation environment in which an excitation voltage exceeds 15V, the life of the phosphor becomes extremely short.
Although the life of the phosphor can be prolonged a little by forming the protection film consisting of the oxide on the phosphor component or adding PtO2 or the like to the phosphor component, the life of the phosphor is not so long as to be practically used. The life of a mixed type phosphor using SrTiO3: Pr, Al for low-voltage electron beams, is not so long as to be practically used, the mixed type phosphors cause easily color changing and a light emitting unevenness, because the life of SrTiO3: Pr, Al is short.